1. Field of the Invention
The present invention relates to detection of head jitter in a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
The disks are rotated by a spindle motor of the drive. Rotation of the disks creates an air flow within the disk drive. Each head has an air bearing surface that cooperates with the air flow to create an air bearing between the head and the adjacent disk surface. The air bearing eliminates or minimizes the mechanical wear between the head and the disk. The height of the air bearing is commonly referred to as the flying height of the head.
The magnetic field detected by the head is inversely proportional to the flying height of the head. Likewise, the strength of the magnetic field written onto the disk is also inversely proportional to the fly height. A larger fly height will produce a weaker magnetic field on the disk.
There have been developed disk media that have patterns of magnetic dots. Such disks are commonly referred to as bit patterned media. The dots are constructed from magnetic material and are separated from each other by non-magnetic material. The non-magnetic materials inhibits cross-talk between the dots. The write clock must be very accurate so that the writing of data occurs above a magnetic dot.
It is desirable to create a flying height that is nearly zero. A nearly zero flying height can result in contact between the head and disk. Contact between the head and the disk can cause vibration and associated head movement. Such movement can destroy synchronization between the write clock and the writing of data in a bit pattern media. It is therefore desirable to detect vibration induced movement of the heads.